1. Field of the Invention
The present invention relates to a light emitting device that regulates a light distribution angle and illuminates a lighting field with high uniformity.
2. Description of the Related Art
To concentrate light, which is radiated from a light source, for illuminating a lighting field efficiently is important for designing a light emitting device. Various light emitting devices have been developed to work out such issue. (For example, see Patent Document 1: Japanese Patent Application Laid-open No. 2006-128041.)
As shown in FIG. 8, a light emitting device 1 described in the Patent Document 1 comprises mainly a surface light source 2; a bowl-shaped reflector 6 having an opening for incidence 3 through which the light from the surface light source 2 passes inside; an opening for radiation 4 facing to the opening for incidence 3; and a reflecting surface 5 reflecting the light from the opening for incidence 3 to the opening for radiation 4.
When a plane M containing a center axis CL is assumed, two points of peripheral border 7a, 7b of the opening for incidence 3 and two point of peripheral border 9a, 9b of the opening for radiation 4 are determined. And a coordinate system having a horizontal axis passing through the point of peripheral border 7b, and parallel to a straight line X passing through the points of peripheral border 7a and 9 is determined. The reflecting surface 5 of the reflector 6 is a paraboloid of revolution that is determined by revolving a parabola 8 around the center axis CL. The parabola 8 passes through the points of peripheral border 7a and 9a, and has a focal point on the other point of peripheral border 7b of the opening for incidence 3.
According to this light emitting device 1, the light from the surface light source 2 is radiated within an aperture angle θ that is determined with a straight line X passing through the points of peripheral border 7a, 9b and a straight line Y passing through the points of peripheral border 7b, 9a. For example, when the light emitted from the point of peripheral border 7b of the opening for incidence 3 directly passes through the opening for radiation 4 without reflecting on the reflecting surface 5, the light forms an angle with the center axis CL. The angle is the same angle between the straight line Y and the center axis CL (=θ/2). In contrast, the light reflected on the reflecting surface 5 is parallel to the straight line X, because the point of peripheral border 7b is on the focal point of the parabola 8. All light from outer edge of the opening for incidence 3 is in the same manner, because the outer edge of the opening for incidence 3 is determined with a circle that is around the center axis CL of the point of peripheral border 7b. The point of peripheral border 7b is on the focal point of the parabola 8 in the plane M. The light that is radiated from inner side of the opening for incidence 3 is radiated in an angle. The angle between the light and the center axis CL is smaller than ½θ.
Therefore, a lighting area S of the light from the surface light source 2 is regulated within predetermined angle θ by the reflector 6.